From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 98502385507 for ; Thu, 2 Apr 2026 10:33:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775126009; cv=none; b=NM7wR+l1sIoBM2lwLBvGJeM2Ub8WgBKiRHACq3hX4sis7abgkAWSU2E9vGUCrBsnCmZdI44oiQrEvreTOKcHQp/JDSg5wLyKkogJBt2MMM7X9cmW35BvZ8gfY1uccty8eBMa5d8jbEkmVGkgdx92cuIKFIcQiNg9OduSOSg/SzQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775126009; c=relaxed/simple; bh=11qgtPGLwVASO1RXj7Dpucv+msw2zwj+vSe9j1rvdlE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=haQn8OpsKhLVJY50TktI1DzPm1+T2AlJz7LAat3FLXPfCN7dPSqNXuKgWquoYVDXiWR8koEqDEkcztlnaLMEPbuCF5G+D4VkgYSYuedfY96vvI/N1zOrkjJujnz88qN7bY1gr+LJ/EFt8foin7F2QbfP6KSzOZqD+ocyE0oJjaY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UJ8fJQl8; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="UJ8fJQl8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 78B6DC116C6; Thu, 2 Apr 2026 10:33:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775126009; bh=11qgtPGLwVASO1RXj7Dpucv+msw2zwj+vSe9j1rvdlE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=UJ8fJQl8n4Pd8K3AaUXqGdyXtNvdexXPyW4PaHzM4fY7VzdrgJRAy47fOM1wWrdid tWgHgMOs1vZt8xg3uV4lS5WOmKYJIhEERwZMTfE3KK54c+Q+KNqqNniYNFtc+cMj8M NsHj6pQ+fKcL+Sv3nRUlc6mprAXBd2aUBF/65dXBgyyOmQEp9IMXASWnctZgpenCfM u4I2vXn/oo/d+/ynfi/wASHXzi9SVHoIKR61JETJlOnqiF6S9EHBbZzldZdX5zB/hh 3tAY/z3bJHaecdfw4XQUKDjb5ReSX0pJcSe0IxJCQn1ijU0hf1wkIIaGPEBHUQmdEw 97RJ6aBE9HXRw== Message-ID: Date: Thu, 2 Apr 2026 12:33:23 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/sparse: fix comment for section map alignment To: Muchun Song , Andrew Morton Cc: Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Petr Tesarik , linux-mm@kvack.org, linux-kernel@vger.kernel.org, muchun.song@linux.dev References: <20260402102320.3617578-1-songmuchun@bytedance.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260402102320.3617578-1-songmuchun@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/2/26 12:23, Muchun Song wrote: > The comment in mmzone.h currently details exhaustive per-architecture > bit-width lists and explains alignment using min(PAGE_SHIFT, > PFN_SECTION_SHIFT). Such details risk falling out of date over time > and may inadvertently be left un-updated. > > We always expect a single section to cover full pages. Therefore, > we can safely assume that PFN_SECTION_SHIFT is large enough to > accommodate SECTION_MAP_LAST_BIT. We use BUILD_BUG_ON() to ensure this. > > Update the comment to accurately reflect this consensus, making it > clear that we rely on a single section covering full pages. > > Signed-off-by: Muchun Song > --- > v1 -> v2: > - Drop the actual BUILD_BUG_ON logic modification (keeping the simple > comparison) and only simplify/clarify the mmzone.h comment. > - Add explanation explicitly noting that a single section is always > expected to cover full pages, per discussions with David Hildenbrand > and Andrew Morton. > --- > include/linux/mmzone.h | 25 ++++++++++--------------- > 1 file changed, 10 insertions(+), 15 deletions(-) > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 7de42be81d4b..a071f1a0e242 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -2056,21 +2056,16 @@ static inline struct mem_section *__nr_to_section(unsigned long nr) > extern size_t mem_section_usage_size(void); > > /* > - * We use the lower bits of the mem_map pointer to store > - * a little bit of information. The pointer is calculated > - * as mem_map - section_nr_to_pfn(pnum). The result is > - * aligned to the minimum alignment of the two values: > - * 1. All mem_map arrays are page-aligned. > - * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT > - * lowest bits. PFN_SECTION_SHIFT is arch-specific > - * (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the > - * worst combination is powerpc with 256k pages, > - * which results in PFN_SECTION_SHIFT equal 6. > - * To sum it up, at least 6 bits are available on all architectures. > - * However, we can exceed 6 bits on some other architectures except > - * powerpc (e.g. 15 bits are available on x86_64, 13 bits are available > - * with the worst case of 64K pages on arm64) if we make sure the > - * exceeded bit is not applicable to powerpc. > + * We use the lower bits of the mem_map pointer to store a little bit of > + * information. The pointer is calculated as mem_map - section_nr_to_pfn(). > + * The result is aligned to the minimum alignment of the two values: > + * > + * 1. All mem_map arrays are page-aligned. > + * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT lowest bits. > + * > + * We always expect a single section to cover full pages. Therefore, > + * we can safely assume that PFN_SECTION_SHIFT is large enough to > + * accommodate SECTION_MAP_LAST_BIT. We use BUILD_BUG_ON() to ensure this. > */ > enum { > SECTION_MARKED_PRESENT_BIT, Thanks! Acked-by: David Hildenbrand (Arm) -- Cheers, David